This invention relates to methods and apparatus for geophysical exploration and other subterranean investigations. More particularly, it relates to an apparatus with which magnetic anomalies of temporal and spatial origin may be measured simultaneously and conveniently distinguished. The apparatus enables several electrical and magnetic parameters of the ground to be determined and mapped in the one operation and with the one sensing device. The speed with which this apparatus can obtain each measurement is such that it has now become practical to routinely record measurements at intervals of a meter or less and by so doing achieve exceedingly high definition in near surface geophysical investigation. Such information can be useful in geological mapping, and in the location of such things as mineral deposits, groundwater and petroleum and buried artificial items such as pipe-lines, explosive ordnance, archaeologically valuable material and the like.
Geophysical exploration methods involve the measurement of physical properties which vary in a manner which is related to changes in the composition and structure of the ground in the area of investigation. Such properties include, density, magnetic susceptibility and remanence, seismic velocity, electrical conductivity and polarizability and many more. Geophysical literature describes various apparatus for individually measuring these parameters. Two families of these physical properties are commonly measured in geophysical exploration methods. These are:
1. Magnetic Properties PA1 2. Electrical and Electromagnetic Properties
The earth's magnetic field varies both in space and time. It is well established in the prior art to make use of this magnetic field in a number of ways for geophysical investigations. For example, magnetic detectors have been most commonly used to determine the magnitude of the earth's magnetic field at a number of points within a survey area. The spatial variation in the earth's magnetic field over the area surveyed can be isolated by subtracting from each measurement the value of the magnetic field measured simultaneously at a nearby, stationary reference point. The spatial variation in the magnetic field are quantitatively related to changes in the magnetic properties of the ground.
Electrical resistivity, electromagnetic and induced polarization mapping have been described in many articles to be found in geophysical literature. With each of these methods, an electric current is created in the ground from artificial galvanic or inductive sources, or by natural means. The flow of current will behave according to established electrical and electromagnetic rules. For example, it will preferentially flow through material that is of more conductive composition, and if the introduced current is of alternating polarity, then the current flow also obeys a frequency-depth relationship. It is also known that by creating an artificial current in the ground, a polarization effect may occur to a varying degree depending upon the composition of the material present and the frequency-depth relationship.
Various artificial energising current waveforms and frequencies may be used for different effects. Point by point measurement of the potential between electrodes placed in contact with the ground, and measurement of the secondary electro-magnetic field due to the current in the ground, and measurement of the phase between the transmitted waveform and the received signal have all been previously used, either individually or in combination, to map the subterranean electrical properties.
Galvanic measurements have of necessity been made point by point where electrodes were located in the ground. Consideration was required of the geometry of the electrode array. Limitations associated with the use of electrodes are avoided by applying inductive sources to generate the required currents in the ground. Individual components of the secondary electro-magnetic field arising from the flow of current in the ground are then commonly measured with induction coil type receivers. When inductive sources and receivers were employed, the method could be adapted to moving, marine or airborne applications. The use of inductive techniques required that consideration be given to the orientation of the transmitter and receiver.